The resonator fiber optic gyroscope (RFOG) has the potential of meeting the needs of many navigation and rotation sensing markets and creating new markets because of its capability of high performance in a small size, at low power and low cost. FIG. 1 illustrates a conventional RFOG 10 consisting of a clockwise (CW) laser 12, a counter-clockwise (CCW) laser 14, a fiber optic resonator 16 and electronic circuits (“electronics”) providing at least resonator-coupling and resonance-tracking (or resonance-detection) functionality. The CW laser 12 inputs light into the resonator 16 and a CW photodetector 18 detects the CW output of the resonator.
The electronics downstream of the CW photodetector 18, which include a CW modulation generator 20, a CW demodulator 22, a CW accumulator 24, and a summing element 26, control the CW laser frequency to a resonance frequency of the resonator 16. The resonance frequency is detected by modulating the laser frequency at f1 using the CW modulation generator 20 and then demodulating the output of the CW photodetector 18 at f1 using the CW demodulator 22. At the resonance frequency, the CW photodetector 18 signal at f1 passes through zero amplitude. The CW accumulator 24 controls the laser frequency via the CW laser driver 38 to the resonance frequency by adjusting the laser frequency until the output of the CW demodulator 22 is zero. The modulation at f1 is electronically summed with the CW integrator 24 output by the summing element 26. With regard to the similarly configured CCW path of the RFOG 10, the CCW laser 14 is controlled to the CCW resonance frequency in a similar manner, except it is common that the modulation frequency f2 is different than f1 to eliminate errors that arise when light from one direction of propagation in the resonator 16 inadvertently couples into the other direction.
In order to achieve high performance, the RFOG electronics must be capable of digitizing and detecting a very small signal from the resonator 16 and at specific frequency in the presence of a very large unwanted (e.g., harmonic) signal. The required resolution cannot be obtained with conventional analog-to-digital converters (ADCs) alone, and therefore some type of filter, to remove the unwanted signal to allow additional gain of the rotation signal so that conventional ADCs can be used, would be desirable. Ideally, such a filter removes the unwanted signal without removing noise that is required for ADC bit interpolation.